Crosslinked solubility parameters

The above discussion demonstrates that the solubility parameter concept in combination with the gradient oven is a useful tool to select a convenient solvent, which could undergo a phase separation during the crosslinking reaction. 

These highly amorphous elastomers have relatively low Tt values (—73 C) and tend to crystallize when stretched. The cold flow of these thermoplastic polymers is reduced when they are crosslinked (vulcanized) with a small amount (2%) of sulfur. Since these polymers of isoprene have a solubility parameter of 8.0 H, they are resistant to polar solvents but are soluble in many aliphatic and aromatic hydrocarbon solvents. The cross-linked derivatives swell but do not dissolve in these solvents. 

Polymers with solubility parameters differing from those of the solvent by at least 2.0 H, will not dissolve in the solvent at room temperature. Thus although unvulcanized natural rubber (NR), unvulcanized styrene-butadiene elastomer (SBR), unvulcanized butyl rubber, and EPDM dissolve in gasoline or benzene, the vulcanized (cross-linked) polymers are swollen but will not dissolve due to the presence of the crosslinks. 

Phase separation occurs when AG rises above 0. This may be triggered by a rise in enthalpy (i.e., AH) or a decline in entropy (i.e., AS). To allow for the formation of a uniform network polymer, phase separation must be delayed until crosslinking is well enough advanced to prevent individual molecules from demixing. This delay is achieved by either reducing AH or by raising AS (in concert with T). The enthalpy factor (AH) is controlled by the difference in Hildebrand s solubility parameter (5) between the various reacting components, since... 

Although the kinetics of liquid uptake to attain gel-saturation is history-dependent, the composition at the true end-state (i.e. thermodynamic equilibrium in excess liquid) is not therefore the observed end-state is usually reproducible [19]. Gel-saturation is attained when the restraining force (per unit area) of the polymeric crosslinked network becomes equal and opposite to the osmotic pressure that causes the system to swell [20], In other words saturation is achieved when the chemical potential of swelling liquid, p1 in the swollen network is equal to the chemical potential of the excess pure liquid, p , outside the network. It was logical to anticipate that the volume of liquid sorbed per gram of polymer, at this state of thermodynamic equilibrium with excess liquid, would correlate with the molecular structure of the liquid. In fact two parameters already exist which relate the sorption affinity to the molecular structure, namely the solubility parameter, 8, first proposed by Hildebrand [21], and the interaction parameter, %, introduced by Flory [22] and Huggins [23-26],... 

Gee [41] has shown that the same is normally true for swellability of crosslinked polymers, i.e. the magnitude of swelling exhibited by a polymer with 8pol in a liquid with 8]iq increases as (Spol — S,Jq) approaches zero. Such correlations have been used to establish the solubility parameters of many polymers and these data are also collected in extensive tables [27, 28]. 

ABS-type resins are used as impact modifiers for PVC, but the resultant blend has insufficient transparency for application in clear bottles. Transparency can be obtained by grafting PMMA onto crosslinked PBD (5) or poly (butyl acrylate) (6) which has been previously grafted onto PS (Diagram 4). In this case the PMMA branch is compatible with PVC by virtue of its solubility parameter, and optical clarity results from suitable component ratios in the graft copolymer so that the refractive index matches that of PVC. The desired results are not obtained if a copolymer of methyl methacrylate and styrene is... 

When a crosslinked or otherwise insoluble polymer is swollen with a series of solvents of differing solubility parameters 5, a maximum in swelling will occur when the solubility parameter of the solvent roughly equals that of the polymer (Tobolsky, 1960, pp. 64-66). Fritzsche and Price (1971) reasoned that a two-phase insoluble block copolymer should display two independent peaks, and used the swelling method to examine this aspect of their material. Typical results are shown in Figure 6.26. The peak... 

Fig. 9.6 Linear expansion with respect to the solvent solubility parameter (/ ), and optical images of the corresponding instabilities for PHEMA hydrogel with 3 and 1 % crosslinker (EGDMA) (right). (Reproduced frran [34] with permission from The Royal Society of Chemistry)...

Table 5.1.2. Values of solubility parameters for crosslinked elastomers from swelling in symmetric liquids. [Adapted, by permission, from V.V. Tereshatov, V.Yu. Senichev, A.I. Gemuev, Vysokomol. soed., B32, 412 (1990).]...

Table 5.1.3. Values of solubility parameters of crosslinked elastomers from swelling...

The samples of crosslinked elastomers were swollen to equilibrium at 25 C in 11 SLs containing solvents of different polarity. The following regularities were established. With decrease in the solubility parameter value of component 1 (see Table 6.2.1) in SL (8, < ), the maximum value of equilibrium swelling, Q, shifts to the field of larger concentration of component 2 in the mixture (Figures 6.2.1 and 6.2.2). On the contrary, with decrease in the... 

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